Baths suitable for rehalogenating metallic silver in photographic materials



United States Patent 3 264,107 BATHS SUITABLE FdR REHALOGENATHNG ME-TALLIC SILVER IN PHOTOGRAPHIC MATE- RIALS Ren von Warthurg, Basel,Switzerland, assignor to Ciha Limited, Basel, Switzerland, a Swisscompany No Drawing. Filed Aug. 29, 1952, Ser. No. 220,156 Claimspriority, application Switzerland, Sept. 8, 1961, 10,465/61 Claims. (Cl.96-450) In many photographic, and more especially color photographicprocesses there arises the necessity of removing a metallic silver imageor oxidizing it to a metal salt image, for example, without attacking acolor image.

While it is generally easy to remove metallic silver completely fromsingle-layer materials that are free from color couplers or dyestuiis,this is often difiicult in the case of multi-layer or mixed grainemulsion materials containing dyestuffs, because there remains in thearea from which silver has been removed a faint, dark colored residualimage, which may consist of silver sulfide or of silver enclosed bydyestuif, and which is particularly troublesome in the case of images tobe viewed by reflected light.

To oxidize completely silver images in mate-rials that have beensubjected to color development numerous proposals have been made foractivating the usual ferricyanide baths or replacing them by baths of adifferent nature.

The problem of the complete removal of metallic silver arises also inthe silver dye bleaching process in which, as is known, dyestuffs arebleached out in single-layer or multi-layer materials in dependence onthe amount of metallic silver locally present in the photographic image,and any remaining metallic silver must be subsequently removed.

The material to be processed generally comprises a support on which arecoated a plurality of light sensitive silver halide emulsions, which maybe in the form of layers or granules and which contain bleachabledyestuffs in a difiusion-resistant form. After exposure, development andfixation, the dyestufis are bleached in dependence on the amount ofsilver locally present in the silver image. This is generally carriedout in a bath which contains an acid, a substance capable of formingsilver complexes, such as thiourea, and a bleaching catalyst.

When the material is subsequently treated in a known silver bleachingbath, for example, a bath containing copper sulfate, sodium chloride andhydrochloric acid,

the metallic silver is not completely oxidized, or impuritiesaccompanying the silver are not completely removed, so that, aftertreatment in a fixing bath following upon the silver bleaching bath,there is left a residual density which becomes apparent in images to beviewed by reflected light as poor remission in the light areas of theimage. While it is possible to improve the result by repeated alternatetreatments with the silver bleaching bath and the fixing bath, thismethod is very troublesome, so that there is a need for a silverbleaching bath which completely oxidizes the metallic silver in asingle, short treatment.

The present invention is based on the observation that a silverbleaching bath which contains a hydrohalic acid, a cupric salt andwater-soluble aliphatic carboxylic acid converts metallic silver in avery short time and quantitatively into silver halide, even when thesaid silver bleaching bath is used after a dye bleaching bath thatcauses the silver to become less reactive. Accordingly, the inventionprovides for rehalogenating metallic silver in a photographic material,a bath which contains a cupric salt, a hydrohalic acid and awater-soluble aliphatic carboxylic acid.

As the hydrohalic acid there is preferably used hydrochloric orhydrobromic acid. While it is possible to form these acids by doubledecomposition, for example, by incorporating in the bath potassiumbromide and sulfuric acid, the best results have been obtained withbaths which contain no or a very small amount of metal ions, forexample, alkali metal ions, other than the copper ions, and which arepractically free from sulfate ions. Therefore baths which contain,instead of the often used copper sulfate, other copper salts, forexample, copper acetate or copper nitrate, are preferred. The bathsadvantageously contain about 0.5 to 2.0 mols of the hydrohalic acid andabout 0.1 to 1.0 mol of the copper salt per liter.

As aliphatic carboxylic acids there may be mentioned, for example,unsaturated acids and polycarboxylic acids, though other acids maylikewise be used, provided that they contain no basic substituents andare water-soluble. The acids are advantageously composed exclusively ofcarbon, hydrogen and oxygen, and advantageously contain not more than 7carbon atoms. It is of advantage to use acids capable of formingwatersoluble copper compounds.

As examples of such acids there may be mentioned: acetic acid, lacticacid, cyanoacetic acid, acrylic acid, succinic acid, tartaric acid,citric acid, and maleic acid.

As a rule, the best results are achieved with acrylic and citric acid.

The mechanism behind the action of these acids has not yet beencompletely elucidated. It is probable that they act, on one hand, assolvents or solution promoters for the products that passivate themetallic silver, such as dyestuff fission products and thioureacomplexes, and radicals of bleaching catalysts or of filter dyestuffs.However, since, on the other hand, these acids are capable of formingcomplexes with the copper ions, they are able to modify the oxidationconditions. The acids give their optimum effect at a concentrationranging from about 1 to 15%, and preferably 2 to 12%.

Accordingly, the preferred silver bleaching baths of the invention arethose which contain, per liter, about 0.5 to 2 mols of a hydrohalicacid, such as hydrochloric acid, 0.5 mol of a copper salt, such ascopper nitrate, and about 2 to 12% of an aliphatic carboxylic acid, suchas acrylic or citric acid.

The baths can be further improved by adding thereto a small proportionof a surface-active agent, for example, a polyglycol ether of a higherfatty alcohol, and especially a fiuorinated aliphatic carboxylic acidsuch as described, for example, in U.S. specification No. 2,904,515.

The baths of the invention for rehalogenating metallic silver aresuitable for use in all processes in which strongly acid baths are notharmful. They are therefore suitable for use in those color photographicprocesses in which dyestuffs are not decomposed by acid, moreespecially, for the silver dye bleaching process. In the known method ofcarrying out the silver dye bleaching process, in which the silverbleaching bath is applied after an acid dye bleaching bath, the baths ofthe invention are superior to the known silver bleaching baths. Theybring about more rapidly the complete rehalogenation of metallic silveror complete removal of the impurities associated with the silver inmultilayer materials to be viewed by reflected light so that, aftersubsequent removal of the silver halide in a fixing bath, clearerpictures and above all better whites, are obtained.

The azo-dyestuifs suitable for use in the silver dye bleaching processhave in general a good resistance and satisfactory stability to therehalogenation baths of the invention, but there are cases in which thedyestufis are adversely affected when the bath has a very highconcentration of aliphaticcarboxylic acid. In such cases the addition ofan organic base or of a salt thereof to the rehalogenation bath protectsthe dyestuif from oxidation without impairing the action of the bath onthe silver to be rehalogenated or on the dark impurities accompanyingthe silver. Especially suitable bases are biguanides such, for example,as diphenyl-4:4'-dibiguanide, in a concentration below 1%. While bathscontaining no base give in many cases, though not always, satisfactoryresults, baths that contain a base as aforesaid are more generallyapplicable.

The following examples illustrate the invention:

Example 1 A white-reflecting support was coated with a layer of ared-sensitized silver bromide emulsion containing a diffusion-resistantblue-green azo-dyestuff, this layer was coated with a green-sensitizedsilver bromide gelatine emulsion containing a diffusion-resistant purpleaZo-dyestuff, the latter layer was coated with a yellow filter layer,and the filter layer was coated with the topmost layer of anon-sensitized silver bromide gelatine emulsion containing adiffusion-resistant yellow azo-dyestuif.

The material was exposed, developed, fixed and then bleached in a bathcontaining hydrochloric acid, sodium chloride, thiourea and2:3-aminohydroxyphenazine, whereby all dyestuff in the areas that hadbeen exposed to white light were destroyed. The material was thoroughlywashed and bleached for 10 minutes at 24 C. in one of the silverbleaching baths described below, once more washed, fixed, washed anddried. The densities of the exposed areas as viewed by -reflected lightwere then measured.

As a measure of the action of the silver bleaching bath there are givenbelow the values for log I /I for blue light, in which I is theintensity of the incident light and I is the intensity of the remittedlight. The corresponding values for green and red light are smaller, butthey vary relatively to each other in a similar ratio.

(A) For a known bath of the composition:

Sodium chloride gr-ams 100 Crystal copper sulfate grams 100 Hydrochloricacid 37% strength --ml 100 Water to make 1 liter.

the log l /I value was found to be 0.32. The known ferricyanide bathsdid not produce any more favorable results; above, all, they producedspotty images which made accurate measurements impossible.

(B) For a bath in accordance with the invention having the composition:

Hydrochloric acid 37% --ml 90 Copper nitrate, Cu(NO .3H O grams-.. 135Acetic acid -grams 60 Water to make 1 liter.

the log I /I value was found to be 0.24.

In the following Table are given baths C to M of the invention, whichhad the same composition as bath (B) exc pt that the acetic acid wasreplaced by the additives specified. The concentrations specified arethose that gave the optimum results in another series of tests:

Example 2 A material having the arrangement of layers described inExample 1, was made in which the green-sensitized layer contained adyestuif of the formula SOaH lTIH:

Hydrochloric acid 37% strength -ml-.. Copper nitrate .3H O grams-Acrylic acid ml 125 Surface-active agent 0.1% ml 15Diphenyl-4:4'-dibiguanide hydrochloride --grams-- 3 Water to make 1liter.

In the area exposed only to blue and red light the magenta dyestuff hadretained its original color strength, while the color strength of theother area exposed to the same light, after treatment in a control bathof identical composition but not containing diphenyl dibiguanide, wasconsiderably weakened. For the areas exposed to white light, both aftertreatment with the bath containing biguanide and with the control bath,the value for log l /I was 0.16.

What is claimed is:

ll. A bath suitable for rehalogenating metallic silver in a photographicmaterial, which bath is substantially free from alkali metal ions andsulfate ions and which comprises a cupric salt, a hydrohalic acid and awatersoluble aliphatic carboxylic acid composed exclusively of carbon,hydrogen and oxygen.' 7

2. A bath suitable for rehalogenating metallic silver in a photographicmaterial, which bath is substantially free from alkali metal ions andsulfate ions which comprises a cupric salt, a hydrohalic acid, asurface-active agent and a water-soluble aliphatic carboxylic acidcomposed exclusively of carbon, hydrogen and oxygen.

3. A bath suitable for rehalogenating metallic silver in a photographicmaterial, which bath is substantially free from alkali metal ions andsulfate ions and which comprises a cupric salt, a hydrohalic acid, abiguanide base and a water-soluble aliphatic carboxylic acid composedexclusively of carbon, hydrogen and oxygen.

4. A bath suitable for rehalogenating metallic silver in a photographicmaterial, which bath is substantially free from alkali metal ions andsulfate ions and which comprises a cupric salt, a hydrohalic acid andacetic acid. I

5. A bath suitable for rehalogenating metallic silver in a photographicmaterial, which bath is substantially free from alkali metal ions andsulfate ions and which comprises a cupric salt, a hydrohalic acid andacrylic acid.

6. A bath suitable for rehalogenating metallic silver in a photographicmaterial, which bath is substantially free from alkali metal ions andsulfate ions and which comprises a cupric salt, a hydrohalic acid andcitric acid.

7. A bath suitable for rehalogenating metallic silver in a photographicmaterial, which bath is substantially free from alkali metal ions andsulfate ions and which comprises a cu ric salt, a hydrohalic acid, asurface-active agent and acrylic acid.

8. A bath suitable for rehalogenating metallic silver in a photographicmaterial, which bath is substantially free from alkali metal ions andsulfate ions and which comprises a cupric salt, a hydrohalic acid, asurface-active agent and citric acid.

9. A bath suitable for rehalogenating metallic silver in a photographicmaterial, which bath is substantially free from alkali metal ions andsulfate ions and which comprises a cupric salt, a hydrohalic acid, asurfaceactive agent, diphenyl-4:4'-dibiguanide hydrochloride and acrylicacid.

10. In a process for the production of a photographic image by thesilver dye bleaching process, the step which comprises carrying out thesilver bleaching after the dye bleaching in a bath which comprises acupric salt, a hydrohalic acid and a water-soluble aliphatic carboxylicacid.

References Cited by the Examiner UNITED STATES PATENTS 10/ 1938 Rogers204-52 X 8/ 1943 Morris 96-20 6/ 1944 Ehrenfried 96-20 1/1950 Speck96-60 2/ 1954 Mueller 96-20 9/ 1958 Hanover 204-52 9/1958 Lyman et a1.96-60 1/ 1964 Tregillus et a1 96-29 FOREIGN PATENTS 5/1957 GreatBritain. 6/ 1957 Great Britain.

NORMAN G. TORCHIN, Primary Examiner.

A. D. RICCI, C. E. DAVIS, Assistant Examiners.

1. A BATH SUITABLE FOR REHALOGENATING METALLIC SILVER IN A PHOTOGRAPHICMATERIAL, WHICH BATH IS SUBSTANTIALLY FREE FROM ALKALI METAL IONS ANDSULFATE IONS AND WHICH COMPRISES A CURPRIC SALT, A HYDROHALIC ACID AND AWATER SOLUBLE ALIPHATIC CARBOXYLIC ACID COMPOSED EXCLUSIVELY OF CARBON,HYDROGEN AND OXYGEN.